1. Field of the Invention
The present invention relates to an external cavity type wavelength-tunable light source which is employed in the optical measuring technical field, for example.
2. Description of the Related Art
As a light source which is employed in the optical measuring technology, a small light source which has a wavelength stability in a single mode lasing of a narrow spectral linewidth and can tune a wavelength is requested.
As a conventional external cavity type wavelength-tunable light source, an external cavity type wavelength-tunable light source, as shown in FIGS. 7 and 8, for example, has been known.
FIG. 7 is a block diagram showing a configurational example of a conventional external cavity type wavelength-tunable light source.
This external cavity type wavelength-tunable light source comprises an optical amplifier 1, an optical amplifier driving circuit 4, lenses 5, 6 and 7, an optical isolator 9, a diffraction grating 2, an angle adjusting mechanism 51, a wavelength-tunable driving circuit 53, a parallel moving mechanism 52, a position adjustment driving circuit 54 and the like.
The optical amplifier 1 is formed of a Fabry-Perot type semiconductor laser (LD). An antireflection film (AR coat) 1a is coated on one end surface of the optical amplifier 1. The optical amplifier 1 emits light beams from both end surfaces thereof in response to an injection current from the optical amplifier driving circuit 4.
A light beam emitted from an end surface of the optical amplifier 1 on the antireflection film 1a side is converted into a collimated light beam by the lens 5 and then input into the diffraction grating 2.
The diffraction grating 2 is used as a wavelength selecting reflector, and has a function of reflecting a light beam, which has a particular wavelength decided by an incident angle, out of the collimated light beams being input. Also, the diffraction grating 2 and an end surface of the optical amplifier 1 on which the antireflection film 1a is not coated constitute a laser cavity. When the light beam being selected by the diffraction grating 2 is irradiated once again into the optical amplifier 1, laser lasing can be generated.
The lens 6 is arranged on the emission light axis on the side of the optical amplifier 1 where the antireflection film 1a is not coated, and converts the light beam emitted from the end surface of the optical amplifier 1 into the collimated light beam. The emitted light beam being converted into the collimated light beam is input into the optical isolator 9.
The optical isolator 9 is provided so as not to return the reflected light beam being emitted from the output fiber 8 side to the optical amplifier 1. The light beam being transmitted through the optical isolator 9 is converged by the lens 7 and then irradiated into the output fiber 8 as an output light beam.
An optical arrangement of the optical amplifier 1 and the diffraction grating 2, as shown in FIG. 7, is called a Littrow arrangement.
A lasing wavelength by the Littrow arrangement optical system can be given by EQU .lambda.=2.times.d/M.times.sin(.theta.) (1)
where .lambda. is a wavelength selected by the diffraction grating 2, d is a recess interval of the diffraction grating 2, M is a degree of the diffracted light beam, and .theta. is an angle between a normal of the diffraction grating 2 and the emission light axis of the optical amplifier 1 (incident angle onto the diffraction grating 2).
Then, the diffraction grating 2 can be adjusted to have any angle (.theta.) with respect to the incident light axis by the angle adjusting mechanism 51.
The diffraction grating 2 can be rotated to any angle by controlling the angle adjusting mechanism 51 by using the wavelength-tunable driving circuit 53, and thus the wavelength (Bragg wavelength) calculated by Eq. (1) can be changed arbitrarily. As a result, the wavelength can be tuned within a gain range of the optical amplifier 1.
In addition, the diffraction grating 2 can be moved by the parallel moving mechanism 52 in parallel with the incident light axis.
The diffraction grating 2 can be moved in parallel with the light axis of the cavity by controlling the parallel moving mechanism 52 by virtue of the position adjustment driving circuit 54 to then change the lasing wavelength arbitrarily.
FIG. 8 shows another configurational example of a conventional external cavity type wavelength-tunable light source.
This external cavity type wavelength-tunable light source comprises an optical amplifier 1, an optical amplifier driving circuit 4, lenses 5, 6 and 7, an optical isolator 9, a diffraction grating 2, a reflector 3, an angle adjusting mechanism 51, a wavelength-tunable driving circuit 53 and the like.
In this case, in FIG. 8, the same parts as those shown in FIG. 7 are designated by the same reference numerals and their description will be omitted.
An optical arrangement of the optical amplifier 1, the diffraction grating 2 and the reflector 3, as shown in FIG. 8, is called a Littman arrangement.
A lasing wavelength by the Littman arrangement optical system can be given by EQU .lambda.=d/M.times.[sin(.alpha.)+sin(.beta.)] (2)
where .lambda. is a wavelength selected by the diffraction grating 2, d is a recess interval of the diffraction grating 2, M is a degree of the diffracted light beam, .alpha. is an angle between the normal of the diffraction grating 2 and the emission light axis of the optical amplifier 1 (incident angle onto the diffraction grating 2), and .beta. is an angle between the normal of the diffraction grating 2 and the reflection light axis of the light beam reflected by the diffraction grating 2 (reflection angle from the diffraction grating 2).
The diffraction grating 2 is fixed to an optical base platform 10 to have an incident angle .alpha. relative to the emission light axis of the optical amplifier 1.
The reflector 3 is arranged on the angle adjusting mechanism 51, and reflects again the light beam, which has the wavelength irradiated perpendicularly to the reflector 3, out of the reflected light beam from the diffraction grating 2 calculated by Eq. (2), onto the diffraction grating 2.
The angle adjusting mechanism 51 changes the angle of the reflector 3 around a rotation center 51a so as to adjust the reflector 3 at any angle with respect to the reflection light axis from the diffraction grating 2.
Therefore, the selected wavelength can be changed freely by adjusting the angle of the reflector 3 by virtue of a driving system of the angle adjusting mechanism 51, so that the wavelength variation can be performed within a gain range of the optical amplifier 1.
Also, continuous wavelength variation can be carried out without mode hopping by optimizing the positions of the rotation center 51a of the angle adjusting mechanism 51 and the reflector 3.
In the conventional external cavity type wavelength-tunable light sources (FIGS. 7 and 8), a rotation stage, a linear stage and the like are employed as the angle adjusting mechanism 51 and the parallel moving mechanism 52.
However, according to the conventional external cavity type wavelength-tunable light source in which the rotation stage, the linear stage and the like are employed as the angle adjusting mechanism 51 and the parallel moving mechanism 52, precise change of the wavelength has not been able to be implemented because of generation of mechanical backlash.
Further, in order to control the above mechanisms with good precision, a motor with built-in high precision gears is needed in addition to the rotation stage and the linear stage. In this case, there has been such a problem that the wavelength tuning mechanism is increased in size.